The present invention relates to the preparation of diaryl carbonates. More particularly, this invention relates to improved methods for the preparation of diaryl carbonates by carbonylation of an aromatic hydroxy compound in the presence of a catalyst.
Diaryl carbonates are valuable monomer precursors for the preparation of polycarbonates by melt transesterification. One route for the synthesis of diaryl carbonates is the direct carbonylation of aryl compounds in the presence of carbon monoxide, oxygen, and a catalyst system. The stoichiometric reaction was discovered in the mid 1970's and is detailed in U.S. Pat. Nos. 4,096,169 and 4,187,242 to A. J. Chalk, which use methylene chloride as a solvent. Subsequent work focused on making the reaction catalytic in palladium, as well as solventless. See, e.g., T. C.-T. Chang, European Patent Application Nos. 350,697 and 350,700.
Use of a multi-component catalyst "package", or system further realized increases in reaction rates. U.S. Pat. No. 4,187,242 to A. J. Chalk, for example, discloses a catalyst system comprising a Group VIIIB element (ruthenium, rhodium, palladium, osmium, iridium and platinum) or complexes thereof, and a co-catalyst selected from Group IIIA, IVA, VA, VIA, IB, IIB, VIB, or VIIB metals, together with a base. Copper is a preferred co-catalyst. U.S. Pat. No. 4,201,721 discloses a catalyst package comprising palladium, a manganese or cobalt complex, a base and a desiccating agent.
Alternative catalyst systems are disclosed, for example, in U.S. Pat. No. 5,142,086 to King, Jr. et al. comprising palladium, a quaternary ammonium salt, a metallic co-catalyst selected from cobalt, iron, cerium, manganese, molybdenum, samarium, vanadium, chromium, and copper, and an organic co-catalyst selected from aromatic ketones, aliphatic ketones, and aromatic polycyclic hydrocarbons. Further improvements to catalyst systems are disclosed in U.S. Pat. No. 5,284,964 to E. J. Pressman, et al. As disclosed therein, improved carbonylation yields are obtained with a catalyst system comprising palladium or palladium complexes, an inorganic co-catalyst selected from cobalt, manganese, and copper salts or complexes, an organic co-catalyst selected from certain heterocyclic amines, such as terpyridines, phenanthrolines, quinolines, and isoquinolines, and quaternary ammonium or phosphonium halides.
U.S. Pat. No. 5,498,789 to Takagi discloses a recent development in catalyst systems, wherein the system comprises palladium, at least one lead compound soluble in a liquid phase, at least one halide selected from quaternary ammonium halides and quaternary phosphonium halides, and optionally at least one copper compound. Use of a lead co-catalyst suppresses the production of aryl aromatic ortho-hydroxycarboxylates as by-products. Suitable lead compounds include lead oxides, for example PbO, Pb.sub.3 O.sub.4, and PbO.sub.2 ; lead carboxylates, for example Pb(OCOCH.sub.3).sub.2, Pb(OCOCH.sub.3).sub.4, Pb(C.sub.2 O.sub.4), and Pb(OCOC.sub.2 H.sub.5).sub.2 ; inorganic lead salts such as Pb(NO.sub.3).sub.2 and PbSO.sub.4 ; alkoxy and aryloxy lead salts such as Pb(OCH.sub.3).sub.2, and Pb(OC.sub.6 H.sub.5).sub.2 ; and lead complexes such as phthalocyanine lead. All of the foregoing patents and disclosures are incorporated by reference herein.
As mentioned above, recent research directed toward improving the direct oxidative carbonylation of aryl compounds to yield diaryl carbonates has focused primarily on the catalyst system. Attempts to reduce the cost of commercially implementing direct carbonylation has furthermore focused almost exclusively on three areas: minimizing the cost of the catalyst system components; increasing the efficiency of the catalyst system; and efficient reclamation and recycling of the various catalytic components. In accordance with European Patent Application Nos. 350,697 and 350,700 to T.C.-T. Chang, U.S. Pat. No. 4,096,168 to Haligren, U.S. Pat. No. 5,231,210 to Joyce et al, and U.S. Pat. No. 5,284,964, above, an optimized catalyst system requires a palladium source (presently palladium acetate), an inorganic co-catalyst (cobalt di-(salicylal)-3,3'-diamino-N-methyldipropylamine, hereinafter CoSMDPT), an organic co-catalyst (presently 2,2':6',2"-terpyridine), and a bromide source (generally a tetraalkylammonium bromide or hexaalkylguanidinium bromide). The catalyst system is stirred in neat phenol at 100 to 200 .degree. C. while a gaseous carbon monoxide and oxygen mixture of constant composition is introduced at elevated pressure (up to 1600 pounds per square inch).
However, numerous other variables can also affect the cost of carbonylation of aromatic compounds on a commercial scale, including reaction pressure. U.S. Pat. No. 5,399,734 to King, Jr. et al. discloses that reaction at elevated pressure is required to provide commercially acceptable rates and selectivities. King et al. discloses that improved yields are achieved by a mixture of carbon monoxide and oxygen in the reactor at a substantially constant molar ratio and partial pressure. The mixture of carbon monoxide and oxygen comprises from 2 to 50 mole % of oxygen (based on total carbon monoxide and oxygen), and is introduced into the reactor until a pressure of 200 to 3500 pounds per square inch (psi) at 25.degree. C. is reached. The Examples disclose use of 7.1 mole % of oxygen in carbon monoxide, at total pressures of 2800 psi.
Despite the advantages of this approach, maintaining the reaction at increased total pressure, e.g., up to 3500 psi, substantially increases the installed investment cost for carbonylation on commercial scales. Advances which permit the operation of this process at reduced total pressure without deleteriously affecting diaryl carbonate production rate and selectivity are therefore clearly desirable.